Release control is desirable in numerous applications and in various fields. For instance, release control is often utilized or sought after in applications relating to fertilizers, pesticides and pharmaceuticals. In general terms, “release” is used to refer to the exposure of an originally contained agent, substrate or matter, referred to herein as the “substrate material,” to the surrounding environment. The release of the substrate material is facilitated by a releasing medium (such as a solvent) and a releasing process (such as dissolution or biodegradation).
Further, “control” refers to the ability to affect the release of the substrate material. The definition of control includes the manipulation of various release variables, including, but not limited to, the amount of substrate material released and the release rate. Extension of the release control concept to an appropriate application implies that variable release profiles can be attained through adjustment of the release control technique. “Release profile” refers to the correlation between the amount of substrate material released and time.
In addition to permitting variability with respect to the release profile, it is also desirable that the release control technique be both reliable and cost effective. Reliable release control refers to a technique that is not unduly or significantly influenced by environmental conditions (such as temperature, abrasive handling, etc.), thereby inducing an unpredictable release of the substrate material.
Generally, the release control techniques conventionally utilized and employed in various applications have not been found to be fully satisfactory. For instance, these release control techniques are often not conducive to variable or adjustable release control or such variability is limited. Further, these techniques may not be reliable, therefore limiting the ability to attain a predictable release. Finally, these techniques may not be cost effective, thereby inhibiting their widespread usage.
As indicated, release control techniques are applicable to numerous applications and fields. Agriculture represents one such application in which release control has become increasingly important. More particularly, in recent decades, food producers have instituted more efficient farming techniques designed to better utilize agricultural resources. As higher demands are placed on agricultural production, growers have increasingly focused upon improving crop yields. Consequently, by way of example, fertilizers capable of providing crops with critical nutrients have become an integral tool in attempts to optimize crop yields.
Basic fertilizers are comprised of rapidly degradable chemical compounds that are released, almost immediately as nutrient forms suitable for plant uptake. This conversion is generally performed by simple dissolution or natural soil degradation processes. The unabated nutrient release characteristic of these basic rapid release fertilizers tends to have several disadvantages. First, fertilizer costs are typically increased due to the inefficient nutrient supply. Generally, the initial rate of nutrient release from basic fertilizers is much higher than the rate of plant uptake. Consequently, a significant amount of fertilizer nutrients are susceptible to losses (i.e.: wasted) such as soil immobilization, leaching by rainfall, or volatilization into the atmosphere.
Second, basic fertilizers have difficulty achieving optimum plant nutrition. In order to compensate for a lack of release control and nutrient losses from basic rapid release fertilizers, growers tend to rely on high application rates or multiple applications in an attempt to meet crop nutritional requirements. Growers must also adjust fertilizer application rates to account for variable soil conditions or crop demand. Using such practices, it is difficult to ensure crops are neither deficiently nor excessively fertilized. Without optimum plant nutrition, crop yields cannot be maximized.
Third, the resulting need for multiple fertilizer applications tends to increase labor and equipment maintenance costs and operating time. Fourth, besides being unavailable for future stages of plant growth, lost nutrient chemicals can pose a potential environmental hazard. Once present in surface or subsurface drinking water supplies, leached plant nutrients may become contaminants. In the case of nitrogen based fertilizers, volatilization contributes to the amount of NO and NO emissions in the atmosphere. Fifth, excess released nutrients not consumed by environmental losses may actually be toxic to plants, particularly seedlings sensitive to soil chemistry. Such plant damage is generally referred to as crop “burning.”
Therefore, there is a need in agriculture for controlled release fertilizer products and permeable fertilizer compositions capable of addressing the disadvantages of basic fertilizers which tend to have no release control. Specifically, there is a need for fertilizer products and compositions able to provide improved crop nutrition achieved through variable, controlled nutrient supplies capable of meeting disparate crop demands. Ideally, the fertilizer product or composition provides the correct amount of nutrients at the correct rate over all or a portion of the growing season. Further, there is a need to reduce fertilizer costs associated with fertilizer losses and multiple applications. Finally, there is a need to reduce environmental damage attributable to fertilizer nutrient losses and crop damage attributable to excessive fertilizer chemical concentrations in the soil.
Various attempts have been made to address the deficiencies of basic fertilizers. Specifically, the fertilizer industry has created numerous modified fertilizer products and compositions, which can be classified under the broad categories of “stabilized fertilizers” and “controlled release fertilizers.” The term stabilized fertilizer is used to refer to a fertilizer amended with a chemical inhibitor designed to slow or suppress the natural soil processes responsible for converting the fertilizer into plant usable nutrients (“Controlled-Release and Stabilized Fertilizers in Agriculture,” Dr. Martin Trenkel, International Fertilizer Industry Association, December 1997, p. 12).
Controlled release fertilizers are generally described as either uncoated or slowly degradable fertilizers or coated or encapsulated fertilizers. Generally speaking, uncoated or slowly degradable fertilizers tend to be chemically modified and rendered more resistant to the natural soil degradation mechanisms. For coated or encapsulated fertilizers, a permeable or porous coating composition is typically added to the surface of solid fertilizer granules in order to slow water infiltration into the soluble nutrient core. Many of the currently commercially available controlled release fertilizers release nutrients in a gradual fashion. That is, they possess release profiles with a slower release rate than basic rapid release fertilizers. However, these fertilizers typically do not utilize any appreciable release control technique. Therefore, release profile variations are difficult or impossible to attain.
Fertilizers possessing controlled release properties without the use of a governing coating are typically classified into three general categories. First, “low or limited solubility fertilizers” include conventional soluble fertilizers that have been chemically modified to produce a new fertilizer compound of reduced solubility. Second, “matrix fertilizers” are comprised of granules including nutrient compounds dispersed throughout a binder or carrier material of typically low nutrient value. Although matrix fertilizers are common, they tend to be of relatively low commercial value. This is largely attributable to the fact that, generally, a substantial quantity of low nutrient value binder is required to form the granule. Consequently, the finished product tends to have a low nutrient quantity per unit weight of fertilizer. Third, “supergranules” are slow release fertilizers provided in the form of large briquettes or sticks and rely upon a low surface area/volume ratio to delay the complete dissolution of the fertilizer. Supergranules provide no appreciable control over nutrient release profiles and are generally used in insignificant quantities.
Urea-formaldehyde is the primary “low or limited solubility fertilizer” in use today. The formation of urea-formaldehyde is achieved by reacting urea with formaldehyde under controlled conditions (temperature, time, pH, etc.) to form methylene urea polymers of various chain lengths. The initial nitrogen release from urea-formaldehyde products is associated with the dissolution of unreacted urea (usually less than 15% of the fertilizer nitrogen content). Before the remainder of the urea in the fertilizer can be released, soil microbes must first break down the polymers, thereby making additional urea available for dissolution. The longer the polymers, the longer the degradation time required to free the urea from the polymer chains. Therefore, some attenuation of the nitrogen release profile can be achieved through varying the decree of polymerization of the methylene ureas. Urea-formaldehyde products are available in granule and liquid forms
However, there are several important disadvantages associated with the use of urea-formaldehyde products. First, urea-formaldehyde products tend to be approximately three to five times as expensive as urea. Second, urea-formaldehyde contains about 38% nitrogen. However, some of the nitrogen contained in very long polymers may be released after the growing season, or not at all. Finally, formaldehyde is a toxic material. Health concerns associated with the handling of formaldehyde in production processes and the usage of products made from formaldehyde, have been raised.
Two additional low or limited solubility urea-based fertilizers are also known. The first is isobutylidene diurea (IBDU®—32% nitrogen). IBDU® is formed via a reaction with urea and isobutyraldehyde, resulting in the formation of a single oligomer (very short chain polymer). The release rate of IBDU® is largely influenced by its particle size, where a smaller granule corresponds to a faster release rate. The second is crotonylidene diurea (CDU®—32.5% nitrogen). CDU® is a low solubility urea compound formed by a reaction of urea and acetic aldehyde. As with IBDU®, the nitrogen release rate of CDU® is determined largely by particle size.
Commercially available “matrix fertilizers” typically employ the use of degradable polymer matrices to carry nutrients such as nitrate, phosphate and potassium compounds. The matrix approach is seldom used for highly concentrated fertilizers, such as urea, because the carrier material may comprise as much as 40%, by weight of the total fertilizer. Generally, only low-grade fertilizers, such as NPK 10-10-10 (nitrogen—10%, phosphorous—10%, potassium—10%), are produced using the matrix approach.
Generally speaking, the various materials typically indicated to be suitable for fertilizer matrices are low solubility, insoluble or degradable substances, such as elemental sulphur, manure, apatite (calcium phosphate crystals), rock fines (and other minerals) and thermoplastic resins and cellulose. These low nutrient value matrix materials can comprise 10–90%, by weight of the “fertilizer.” However, the majority of these matrix fertilizers are of relatively low-value. In addition, the matrix fertilizers may provide slower release than a low solubility fertilizer but they do not typically have the capacity to maintain a significant release of nutrients over extended periods of time. The slow release properties of matrix fertilizers result from the fact that the matrix must be (dissolved/degraded or water must migrate through the matrix to release the nutrients contained. As such, the matrix approach to slow release fertilizers provides limited control of the nutrient release rate.
U.S. Pat. No. 4,589,903 (Sato et. al.) describes a process involving dissolving synthetic wollastonite in concentrated sulphuric acid and blending the solution with various types of manure. The mixture is granulated and allowed to ferment. The low quality pellets can then be applied as fertilizers containing relatively low quantities of nutrients and a large number of beneficial microorganisms. The wollastonite and manure form the matrix of the granules.
U.S. Pat. No. 5,653,782 (Stern et. al.) describes a process by which fertilizer particles are preheated to a temperature in excess of the melting point of sulphur, prior to being mixed with solid sulphur prills. The superheated fertilizer melts the sulphur, and as the mixture is agitated in a pugmill, the fertilizer is “coated.” Although the term coating is used extensively throughout the patent, it only refers to coating the particles prior to agglomeration. The resulting fertilizer is comprised of fertilizer particles contained in a sulphur matrix. Accordingly, this process is only suitable for coating those fertilizers capable of withstanding temperatures in excess of the melting point of sulphur (120° C.) in a range of 130–280° C. Many fertilizers would melt or volatilize under such conditions. Urea, for example, melts at 132° C.
Further, the matrix fertilizer may contain a fibrous medium able to absorb water into the granule core thereby dissolving and releasing nutrients (or a herbicide) carried by the fibres. The fibrous material may be an organic medium (cellulose). U.S. Pat. No. 5,471,786 (Clausen) describes the use of a fibrous medium containing a mineral. The mineralized organic material is lignite, consisting of peat (organic) and carbonaceous mineral (coal). The hydrophilic properties of the ligite make the product a suitable plant growing medium. The “planting blocks” are capable of retaining moisture even in conditions of dry soil and low water table.
Finally, absorptive cellulose fibers may be impregnated with plant nutrients and the resulting fibers subsequently bound in a matrix. Once placed in the soil, the moisture and nutrients stored in the fibers may be released. Some of these products may possess degradable coatings in order to prevent premature leaching, but they are not designed to regulate the release of nutrients. As such, these products may have some slow release properties but without the ability to significantly adjust the release profile. The “fertilizers” produced also contain low quantities of nutrients per unit weight, due to the presence of large quantities of carrier fibers and binders.
In contrast, coated or encapsulated fertilizers involve the application of a coating to a substrate material typically comprised of a solid, granular fertilizer. In practice, encapsulated fertilizers tend to be classified according to the composition of the coating. The most commonly used coating compositions are sulphur, synthetic polymers and a combination of sulphur and synthetic polymers.
Current commercially available sulphur coated fertilizer (“SCF”) generally consists of a water soluble or degradable fertilizer encapsulated by a sulphur coating, a sealant coating and typically a conditioner. Although the nutrient release from SCF tends to be slower than the release from basic fertilizers, the initial rate of nutrient release is often still considered to be too rapid. Therefore, it is desirable to obtain better control over the release profile of SCF.
The release mechanism for SCF is typically water infiltration through pores and cracks in the sulphur coating. There tend to be two sources of the imperfections encountered in sulphur coatings. First, properties inherent in molten sulphur introduce defects within the sulphur coating. The fertilizer coating process basically involves spraying a granular substrate material with an atomized spray of coating material. As the fine coating droplets strike the substrate particles, they spread-out and freeze over the granule surface. A description of a typical coating process is provided in U.S. Pat. No 3,991,225 (Blouin). The relatively high surface tension and viscosity of molten sulphur may result in less than ideal granule wetting and coverage, thereby inducing a portion of the coating imperfections.
Second, the formation of additional coating imperfections is attributable to the allotropic nature of sulphur crystals. At various points during the freezing of molten sulphur and the aging of solid sulphur, a variety of atomic structures may be present. These sulphur structures include polymeric, amorphous, monoclinic crystalline and orthorhombic crystalline sulphur. As differential, physical variations in the structure of sulphur occur, imperfections (voids and fissures) of various sizes are formed between the sulphur crystals. Additional cracks and voids are formed as the sulphur crystals are subjected to thermal changes, resulting in differential expansion and contraction between the crystals. Although the amount and formation rate of the defects within sulphur can be influenced by the thermal history of the material, the formation of crystalline sulphur and therefore imperfections, tends to be inevitable.
Increasing the sulphur coating thickness does not provide effective control of nutrient release as imperfections form regardless of coating thickness. In the case of commercially produced SCF, an increase in coating weight does have the effect of decreasing the overall nutrient release. However, the reduced release is simply a result of more fertilizer granules receiving a heavier, layered coating which does not allow any nutrient release within the growing season (termed “lock-off”) Products containing a significant number of “locked-off” granules are inefficient as more fertilizer must be applied to achieve the total desired nutrient quantity.
Therefore, in the case of conventional SCF, control over the coating process may minimize the number of major coating defects, but there is no effective method of accurately controlling the formation of crystal imperfections. Due to a lack of imperfection control, the permeability of the sulphur coat cannot be significantly varied. Consequently, sufficient attenuation of the nutrient release profile is not possible with conventional sulphur coating technologies.
In an attempt to reduce the initial rate of nutrient release, a sealant may be added to the surface of the sulphur coating. The sealant fills the coating imperfections that would otherwise transmit water into the fertilizer granule core relatively quickly. The sealants selected are typically hydrophobic waxes, oils, polyethylene resins or combinations thereof. These temporary sealants are subject to being degraded by soil microbes prior to water penetration through the sulphur coat and into the fertilizer core. Thus, a microbiocide is often applied to the sealant in order to prevent premature degradation of the sealant. As such, sealants act to only delay water contact with the sulphur coating. In addition, sealants often only partially survive typical fertilizer handling operations, resulting in a discontinuous encapsulation of the sulphur coating.
In addition, in order to obtain a relatively free-flowing product that may be easily handled, conditioners mass also be added to SCF Conditioners are typically minerals such as finely divided clay or diatomaceous earth, which counteract the “stickiness” of the sealant
SCF may also be undesirable due to the fact sulphur is a brittle material. Even well formed coatings are prone to cracking and chipping during fertilizer handling operations. In the event the sulphur coating remains intact after handling, the micro-pores and fissures within the coating are generally enlarged, resulting in further degradation of any release control properties.
The insufficient ability to control the release of the substrate material from SCF has resulted in release profiles which are not ideal or even desirable for many applications. This deficiency is exacerbated by the poor coating durability exhibited by conventional SCF. Thus, in summary, SCF lacks desirable performance attributes. First, significant control over a generally undesirable nutrient or substrate release profile is typically not attainable using conventional sulphur coatings. Second, tropical fertilizer handling operations damage relatively fragile sulphur coatings of SCF, resulting in a release profile that tends to be undesirable, unreliable and invariable.
In the case of SCF, recent technological developments have focussed upon improving the sulphur coating durability and/or the coating process. For example, U.S. Pat. No. 4,636,242 (Timmins) describes the modification of elemental sulphur using a dialkyl polysulphide plasticizer. Timmins indicates that the admixture is capable of reducing the viscosity of molten sulphur (resulting in better granule coverage) and plasticizing the solidified coating (resulting in a more flexible coating). These developments may somewhat reduce the rapid, initial nutrient release associated with conventional SCF and improve the handling characteristics of the coated fertilizer as compared with SCF. However, no significant release control technique appears evident.
Synthetic polymer coated fertilizer (“PCF”) is typically comprised of solid fertilizer particles as the substrate material surrounded by a polymer coating (i.e.: polyethylene, polyurethane, polyolefin, alkyd resin, etc.). The are several advantages of PCF as compared to SCF. First, PCF typically possesses a less rapid, initial rate of release and sustained nutrient supply longer into the growing season. Second, polymer coatings are typically more durable than sulphur coatings and therefore, less susceptible to damage during handling. Third, due to the lighter coating material, PCF usually possesses a higher nutrient content, by total weight of fertilizer. In the case of commercially available SCF, the sulphur coating may comprise up to 30% of the total fertilizer weight. By comparison, PCF seldom contains more than 15% coating material, by weight of fertilizer.
However, there are some disadvantages associated with PCF. There may be environmental concerns. Polymer coatings may breakdown very slowly (or not at all), resulting in a plastic residue in the soil system. Further, due to increased process and material costs, PCF is generally significantly more expensive than other controlled release fertilizers, including SCF.
Water infiltration through the porous or permeable polymer coat provides the release mechanism for PCF. Depending upon the technology, the porosity or permeability of the polymer coating may be fixed or variable. In the case of fixed porosity or permeability coatings, no significant control over the nutrient release profile is attainable. A degree of nutrient release attenuation can be achieved with variable permeability polymer coatings. However, due to complex manufacturing processes and expensive materials, the high cost of these products often prohibits their usage in agriculture. The largest market for PCF tends to be horticulture and “high-end” lawn fertilizers.
Commercially available synthetic polymer and sulphur coated fertilizers (“PSCF”) typically include approximately 15% sulphur coating and less than 2% polymer coating. Sulphur is the primary fertilizer coating used in conjunction with the secondary polymer coating which is designed to act as an improved sealant. Polymer sealants are typically more durable than traditional sealants and they do not require the addition of a conditioner to the coated particles.
PSCF is an attempt to combine the lower initial rate of release and durability of polymer coatings with the low-cost of a sulphur coating. The release profile of most PSCF is still predominantly governed by the primary sulphur coating. The polymer topcoat is generally provided to limit degradation of the sulphur coating during handling. More expensive PSCF may incorporate a polymer coating capable of providing a degree of release control (i.e.: a variable permeability membrane).
Further, in the field of construction materials (such as sulphur concrete and the like), the addition of filler materials, including mineral fillers and fibers, to elemental sulphur has been used to create materials with highly desirable “permanent” durability. For example, U.S. Pat. No. 4,484,950 (Hinkebein) discloses an invention in which mixtures of molten sulphur and crystalline phosphate fibers are cast into various structures. The focus of Hinkebein is to provide a strong, durable material suitable for such long-term applications as tanks, pipes and pavement.
U.S. Pat. No. 4,026,719 (Simic) describes a material comprised of sulphur, sulphur plasticizer (such as dicyclopentadiene) and a reinforcing filler such as mica, talc (platy silicates) or glass fibers. The composition is described as useful for durable coatings for floors and slabs. Simic also refers to the potential use of the composition for “water impoundment” applications (such as lining irrigation ditches), thereby implying an impermeable (or very low permeability) material is produced.
In the above mentioned references and others similar in nature, fibrous materials may be used to mechanically reinforce the properties of sulphur compositions in an extreme fashion (i.e.: ultimate strengthening and durability, minimizing or eliminating permeability, etc.). Therefore, it is feasible that filler reinforcement could improve the durability of controlled release products or compositions. However, direct application of the reinforcing techniques described would likely result in an impermeable (or unacceptably low permeability) composition or controlled release product, thereby “locking off” the substrate material.
“Stabilization” is used herein to refer to methods designed to reduce the formation of defects (voids and fissures) at the material crystal level, as described above. Such defects may be formed as a result of differential crystal movement caused by allotropic crystal conversion and/or thermally induced expansion and contraction of the crystals. Stabilization techniques for materials such as sulphur may be classified as chemical stabilization or physical stabilization.
Chemically stabilized sulphur has been used in various construction materials, such as sulphur concrete. According to A. H. Vroom, “Sulphur Polymer Concrete and its Applications,” VII International Congress on Polymers in Concrete, Sep. 22–25, 1992, Moscow, pp. 606–619, a polymeric sulphur concentrate (SRX) is added to molten elemental sulphur. Upon freezing, the SRX polymer is indicated to promote formation of micro sulphur crystals, as opposed to macro sulphur crystals. Apparently, as the modified sulphur experiences crystal conversion and/or thermal changes, less differential movements are experienced by the fine crystals, thereby reducing defect formation.
Dicyclopentadiene, styrene and limonene are examples of polymeric polysulphide plasticizers that, when added to molten sulphur, tend to substantially reduce the amount of crystalline sulphur formed upon freezing (i.e.: more amorphous and polymerized sulphur is present in the cooled material) (B. R. Currell et. al., “New Uses of Sulphur,” Advances in Chemistry Series 140, 1975, pp. 1–17). However, these chemical admixtures generally do not provide permanent stabilizing as sulphur crystals are eventually formed over time.
Polymeric polysulphides have also been used in various sulphur based construction materials such as road markings and masonry coatings. However, in the case of sulphur coated fertilizers, such plasticizing techniques are generally not compatible with the fertilizer coating process. During fertilizer coating, molten mixtures are sprayed onto the fertilizer granule substrate material. Once added to molten sulphur, polymeric polysulphides tend to increase the viscosity and crystallization time of the molten mixture, as described in U.S. Pat. No. 4,129,453 (Simic). Therefore, during spraying, the modified polymeric sulphur tends to exhibit very poor granule wetting and may even agglomerate fertilizer granules, as the modified sulphur requires more time to freeze (R. Jerome Timmins, “Modified Sulphur Coated Urea,” 198th ACS National Meeting, Miami Beach, Fla., Sep. 10–15, 1989, Paper 23, p. 3).
Alternately, fine particulate filler materials have been used to physically stabilize sulphur compositions, primarily in construction material applications. Once dispersed throughout molten sulphur, the particulate inclusions serve as centers for crystallization during freezing, thereby promoting the growth of “uniform, dense, fine-crystal structures” as described in Yu. I. Orlowsky and B. P. Ivashkevich, “Peculiarities of Technology of Production of Sulphur Polymer Concrete . . . ,” VII International Congress on Polymers in Concrete, Sep. 22–25, 1992, Moscow, p. 664. The stabilized crystal structure apparently experiences less and smaller defects during differential crystal movement induced by sulphur crystal conversion and thermal expansion and contraction. Therefore, dispersed particulate filler materials in sulphur may reduce the uncontrollable release mechanism currently utilized in coating applications such as SCF (i.e.: voids and fissures).
Release mechanisms for known, or conventional controlled release products and compositions may, be generally classified into two categories. The first category is solvent infiltration through a conductive coating for the substrate material. The second category is solvent infiltration through a conductive matrix including the substrate material.
Regarding the first category of release mechanisms, a soluble substrate material may be encapsulated with a coating possessing pores introduced at the time of manufacturing (for example, SCF or PCF as described in “Controlled-Release and Stabilized Fertilizers in Agriculture,” Dr. Martin Trenkel, International Fertilizer Industry Association, December 1997, pp. 23–26). Upon contact with the coating, the appropriate solvent can enter the core of the substrate material via the pores and dissolve the substrate material, thereby releasing it to the surrounding environment.
By employing this first release mechanism, control over the release rate may only be achieved by varying the porosity or permeability of the coating. However, many existing coating technologies lack the ability to accurately or significantly vary the coating porosity or permeability. Although polymer coatings with a degree of permeability control exist, the high cost of such coatings often prohibit their widespread application. For example, fertilizers coated with variable permeability polymers are seldom used in mass agriculture applications due to the high cost. The high cost of variable permeability PCF is typically a result of relatively expensive coating materials and relatively complex coating processes.
Regarding the second category of release mechanism, fibrous media may be impregnated with soluble substrate materials. For instance, the absorbent fibers may be agglomerated with the substrate, forming a fibrous “matrix.” Appropriate solvents may then migrate throughout the fibers, releasing the soluble substrate material.
U.S. Pat. No. 5,019,564 (Lowe et al) discloses an invention whereby plant fibers are used to absorb organic pesticides prior to being loosely agglomerated into relatively non-friable “granules” Upon exposure to water, the pesticides absorbed within the fibers are released from the “granules” more slowly than pesticides introduced directly to the agricultural environment.
U.S. Pat. No. 5,762,678 (Hiles) describes the development of a soil enhancing complex in which the soft cores of cellulose fibers are digested, resulting in hollow, “micro-capillaries” composed of the cellulose wall material. The processed “micro-capillaries” may then absorb water and plant nutrients within the cellulose tubes and walls. The laden “micro-fibers” are subsequently agglomerated into pellets and coated with a moisture retaining hydrogel. A gelatinous polymer coating is then applied for the purpose of retaining the integrity of the pellet. The contained nutrients may then be gradually released into the soil environment.
U.S. Pat. No. 5,364,627 (Song) discloses a technology wherein the releasable agent is dispersed throughout the cross sections of polymer fibers. This dispersion is accomplished by mixing the agent with the molten polymer, prior to spinning the mixture into fibers. The release of the agent is accomplished via solvent migration through contiguously arranged agent particles contained within the fiber matrix. Should the releasable agent not be arranged contiguously within the fiber, mechanical action (i.e.: chewing) may be required to expose the releasable agent to solvent contact.
In order to achieve release, the “sponge” or “wick drain” matrix approaches described in the above patents, and several others, rely on solvent transmission through channels or openings contained within the fibrous media. While such techniques are conducive to gradual release, and perhaps controlled release, they are generally not suitable for applications such as high nutrient content fertilizers. One of the factors determining the value of fertilizer products is the nutrient content, by weight of fertilizer. When used in slow release fertilizer applications, the matrix approaches previously described appear to result in a low value fertilizer product due to dependence on a large quantity of non-nutrient, carrier fibers and binders.
Finally, the dispersion of fillers within the permeable composition or controlled release product is also relevant. In this regard, U.S. Pat. No. 4,129,453 (Simic) describes a construction material comprised of plasticized sulphur, reinforcing asbestos fibers and dispersing agents, such as talc or mica which aid in achieving dispersal of the asbestos. The dispersing agent is necessary to avoid “lumpiness” of the molten material mixture. Such dispersing agents are not applied directly on the filler. Rather they added to the plasticized sulphur prior to filler mixing.